In order to make a decision between all of the different possibilities is simply more data and better data. There is a mission in the planning stages that will provide Universe observations for theorists to discriminate between the theories with. Until then the mystery remains.
Visual The image above shows how the universe has expanded over time. Scientists have theorized that the accelerated expansion is due to a mysterious dark energy
We are more sure what it’s not that what it is.
In the forms of stars and planets that we see.
Dark clouds of normal matter.
Antimatter because antimatter gives up gamma rays and we don’t see that with the dark matter.
Incredibly large black holes.
It might be…
There are a few possibilities left .
It could still be baryonic matter if it were to be tied up in brown dwarfs or in small highly dense bits of heavy elements.
The most plausible possibility however is that it is not baryonic but rather made up of more exotic particles such as axions or WIMPS. (Weakly Interacting Massive Particles)
The Hubble Space Telescope and Chandra X-ray observatory have captured observations of powerful collisions of galaxy clusters. The observations seem to indicate that a titanic collision has separated dark from ordinary matter. This seems to be similar to the Bullet Cluster showing that the Bullet Cluster is not a unique case.
above, scientists have come up with the composition that we described, ~70% dark energy, ~25% dark matter, and ~5% normal matter. What is dark matter?. "Dark Energy, Dark Matter — NASA Science." NASA Science . N.p., n.d. Web. 12 Jan. 2010. < http://nasascience.nasa.gov/astrophysics/what-is-dark-energy >.